The actual tip speeds really fit into a narrower range than you might guess. When tips get going too fast, you get unwanted aerodynamic effects, and all the designers have that in mind.

Multiply the diameter by the rpm for lots of helicopters, and the results don't vary enormously from model to model.

A 26 foot rotor at 460 rpm and a 37 foot rotor at 325 rpm will give you essentially the same tip speed, near 425 mph (370 knots). If you wanted the same tip speed with a 100 foot diameter rotor, you would spin it only about 120 rpm.

I think the Osprey would be considered an fixed wing aircraft being its forward motion and lift depend on a wing not a rotor blade, but Hey I could be wrong, just my opinion! as far as rotor diameter being slower with greater length if you tried to spin up longer blades faster wouldnt that cause rotor blade deflection and twist? just asking!!